Can you give an example where you MUST share mutable data that cannot be passed as immutable data structures? The only thing I can think about is hardcore numbe

Can you give an example where you MUST share mutable data that cannot be passed as immutable data structures? The only thing I can think about is hardcore number crunching and yes that should not be written directly in elixir or Erlang or something like that but you could always write NIFs for that.